Printed circuit boards are thin plates in which chips and other electronic devices are placed. The chips and other electronic devices are then electrically connected to each other using conductive traces on the circuit board. Typically, mid-plane printed circuit boards have electrical devices attached to both faces or surfaces of the board and have between one and sixteen conductive layers laminated or glued together. As mid-plane boards become more and more complicated, additional conductive layers are needed. In turn, the additional conductive layers require the mid-plane to become thicker and thicker.
Consequently, the vias in the mid-plane that connect the electrical modules on the surface of the mid-plane to the traces on the various layers in the mid-plane become deeper. The thickness of printed circuit boards may become an issue when a via must be plated to meet a particular aspect ratio (the ratio of the thickness of the printed circuit board to the diameter of the via). Typical guidelines from printed circuit board manufacturers are that the aspect ratio should be below 8–12. For example, the maximum mid-plane thickness for a press fit via that employs a 0.6 mm drill and having an aspect ratio of 10 would be 6 mm.
As a result of thicker mid-planes, “deep” vias become increasingly more difficult to plate effectively and any electrical communication that occurs through the via may become compromised. Additionally, the “deep” vias may require that the mid-plane be back drilled. One purpose of backdrilling is to reduce the length of the via which improves high speed signal characteristics. Long vias act as capacitive stubs which degrade signals as transmission speeds increase. As a result, the manufacturing time and costs associated with thicker mid-planes may increase.
Consequently, there is a need for a low cost mid-plane that can be efficiently and effectively manufactured.